A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the ossicles of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window membranes of the cochlea 104. The cochlea 104 is a long narrow organ wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, various types of hearing prostheses have been developed. For example, when a hearing impairment is related to the operation of the middle ear 103, a conventional hearing aid or a middle ear implant (MEI) device may be used to provide acoustic-mechanical vibration to the auditory system.
FIG. 1 also shows some components in a typical MEI arrangement where an external audio processor 100 processes ambient sounds to produce an implant communications signal that is transmitted through the skin to an implanted receiver 102. Receiver 102 includes a receiver coil that transcutaneously receives signals the implant communications signal which is then demodulated into a transducer stimulation signals which is sent over leads 106 through a surgically created channel in the temporal bone to a floating mass transducer (FMT) 104 in the middle ear. The transducer stimulation signals cause drive coils within the FMT 104 to generate varying magnetic fields which in turn vibrate a magnetic mass suspending within the FMT 104. The vibration of the inertial mass of the magnet within the FMT 104 creates vibration of the housing of the FMT 104 relative to the magnet. And since the FMT 104 is connected to the incus, it then vibrates in response to the vibration of the FMT 104 which is perceived by the user as sound.
FIG. 2 shows a functional representation of a normal cochlea 200. The oval window membrane 201 is a flexible tissue across the opening to the fluid filled scala vestibuli 203. Vibration from the footplate of the stapes drives the oval window membrane 201 creating pressure wave vibration in the fluid of scala vestibuli 203. This in turn creates sympathetic pressure wave vibration in the fluid filled scala tympani 204 on the other side of the basilar membrane 205. The pressure wave vibration of the fluid in the scala tympani 204 in turn drives the membrane of the round window membrane 202 with a phase shift of 180 degrees from the vibration of the oval window membrane 201.
Patients suffering from otosclerosis have serious ossification of their vibrating structures in the middle ear (e.g. ossicles) and in most cases also the membrane of the oval window membrane 201. Consequently, these patients have a severe conductive hearing loss. One problem in connection with an ossified oval window membrane 201 is that the stapes foot plate cannot forward incoming acoustic sound in form of pressure waves into the fluid inside the cochlea 200. In the case of an entirely ossified oval window membrane 201, these patients can be completely deaf even if neural tissue in the cochlea 200 as whole is healthy.
To overcome this problem one could consider mechanically or acoustically stimulating the round window membrane 202 instead of the oval window membrane 201. FIG. 3 shows an example of one approach to round window membrane stimulation where a mechanical middle ear stimulator, e.g., a floating mass transducer (FMT) 301 is placed with its flat front side directly in contact with the tissue of the round window membrane 202 so that movement is not possible between them. This can be achieved by tightly pressing the FMT 301 towards the round window membrane 202 and fixing it there with material produced naturally in the body. Electrical drive signals are delivered from the connecting cable 302 to the FMT 301 which in turns drives the round window membrane 202. Preferably the FMT 301 is placed in the center of the round window membrane 202 where the tissue has its greatest possible elongation.
This method has been used for many patients and is an efficient method to treat hearing disorders for patients lacking of portions of middle ear ossicles. However, because the cochlear fluid is incompressible, a movement of the round window membrane 202 requires a corresponding movement of the oval window membrane 201. But that is not possible when the oval window membrane 201 is immobilized due to ossification. So unfortunately, the arrangement shown in FIG. 3 is not suitable in patients suffering from otosclerosis. Nor can so called third window membranes in the cochlea 200 properly compensate the movement of an ossified round window membrane 202 or oval window membrane 201.
One existing treatment for patients suffering from severe ossification of the middle ear structures such as an ossified oval window membrane uses a so called stapetectomy where a small hole is drilled into the stapes foot plate. A mechanical actuator then is inserted through this hole into direct contact with the cochlear fluid to deliver pressure waves into the cochlea. However, opening and maintaining a permanent hole in the stapes footplate is dangerous due to increased infection risk. Other disadvantages are described in Lupo et. al., Prospective Electrophysiologic Findings of Round Window membrane Stimulation in a Model of Experimentally Induced Stapes Fixation, Otology & Neurology 2009, pp 1-10; which is incorporated herein by reference.
The same paper by Lupo et al. presents a novel method for treating patients suffering from otosclerosis. A ball shaped electrode with a diameter of 1 mm is used on top of a transducer which mechanically stimulates the round window membrane when at the same time the oval window membrane is fixed. The authors further report on the measurement of the amplitude of the Cochlear Microphonic (CM) signal, of the Compound Action Potential (CAP) signal and of the Auditory Brainstem Response (ABR).